The Research of CFD applications for thermal fluid simulation of. Building Environment

Transcription

1 The Research of CFD applications for thermal fluid simulation of Building Environment W.L.Yao 1, Q.Y.Fan 2,Y.W.Zhang 3 and Y.B.Xue 4 1 Shanghai HONGYUAN information technology Co.,Ltd, Shanghai ,China. 2 School of engineering, Osaka University, Osaka Japan. 3 China Academy of Building Research Shanghai Branch, Shanghai ,China. 4 Shanghai Union Building Technology Co.,Ltd, Shanghai, China. ABSTRACT: This paper gives architectural design personnel the guideline in the design phase of the buildings through analyzing two case studies, one is fluid simulation for buildings in a city, the other is thermal fluid simulation for buildings in a residential area. These simulation data are helpful to the actual building layout, green building and choosing building materials. This paper introduces the CFD simulation application in domestic and international advanced design. Especially, this paper introduces the professional building analysis module and the corresponding feature in the relevant software. These modules and features can help designers to finish building environment CFD simulation more conveniently and rapidly. So designers can finish more building CFD simulation in a short-term, and at the same time, designers can improve the building design. These advanced technology includes:solar radiation used for steady-state analysis; the application of multiple grid technology for complicated buildings;. The commonly used evaluation indexes for foreign building CFD simulation analysis are introduced in this paper, for example: MRT, PMV, SET, Index for Age/Life Expectancy of Air, Inlet/outlet Contribution Rate, Ventilation efficiency and so on. KEYWORDS architecture numerical simulation, green environment design, solar radiation, the grid technology, ventilation efficiency, Index for Age of Air 1.Introduction Urbanization construction in China recent years led to series problems, one of the most important of them is the phenomenon called city heat island. For example, xujiahui district in Shanghai, China, which with lots of tall buildings, The average temperature is about 1 degree above higher than Shanghai suburban county.(j.q.zhu,2006) In order to alleviate and reduce the urban heat island influence to climate of the city micro environment, it s necessary to consider the environment condition around the building in city building or building planning design stage. The computational fluid dynamics (CFD) can help designer to optimize design in the early, for example promoting the wind hot environment around building. This article

2 introduced a special CFD analysis tool for construction using, which shows the application of analysis in the construction industry. 2. A CFD simulation technology which is suitable for the construction industry. CFD is short for Computational Fluid Dynamics, which uses the computer as a tool, applies various discrete mathematics method, through the computer numerical calculation and image displays, including complex system analysis as the Fluid flow,heat transfer and other related physical phenomenon (F.J.Wang, 2002). It is an numerical experiment relative to the physics experiment. It s needed to do CFD analysis by using general CFD software to achieve in the actual engineering, but for the CFD software using in the construction industry, which shouldn t only include the basic functions of general CFD software, but also include the construction industry related modules. There is another consideration for professional background of building designer and the high strength work of building designer, the Simple accessibility of the software is very important. The scstream is based on structured grid and finite difference method, with high precision and high calculation speed. This article will show several real architectural engineering, to state that special module of this tool for construction industry and convenience for using. 3. Numerical simulation for complex wind environment of building Numerical simulation results and forecast of wind field around buildings is the foundation for solving other problems, including the ventilation of High-rise buildings around in city, Internal and external ventilation of a building, the emission and distribution of contamination in building surrounding environment. There have been some cities put forward specific requirements for the numerical simulation building of wind environment, such as the 1.5 m plane of the wind velocity distribution and pressure distribution near ground, pressure difference between windward side and leeside of a building and other evaluation indexes (as Zhejiang local standards). The wind speed of building wind environment entrance expresses distribution gradient, for a result of the ground influence, as shown in figure 1 below. To do a practical complex wind environment simulation analysis, after getting the 3-d building model, first step is to create solving field, and set wind speed and temperature calculation conditions according to the local meteorological data, then you need to set the wall friction factor on boundary conditions. In Figure 2, scstream architectural special analysis module can help building designer to fast and accurate to do above steps, efficiently completes analysis. Choose this building wind environment analysis module, which can set wind direction and wind speed after specified conditions (figure 3). it means the rest of the calculation conditions can be set automatically. For example, in a practical engineering, you just only need to set the winter wind direction for NW, wind speed for 3.8 m/s, the rest can be set by software module automatically. This project has 2.8 million grids and the analysis result as follows: partial pressure in windward side is more than 10 Pascal, average surface pressure difference between windward side and leeside is about 5 Pascal, as shown in figure 4; The speed distribution of the 1.5 m plane near the ground is uniform, average velocity is between

3 1 m/s and 2 m/s, pressure distribution is uniform and ideal as figure 5 and 6. In this case, the time of engineers operation is 10 minutes (exclusive 3-d modeling time), calculating time is 34 minutes. Figure 1 wind velocity distribution Figure 2 condition configure interface Figure 3 interface for inlet/outlet setting Figure 4 pressure distribution Figure 5 pressure distribution for z=1.5m Figure 6 velocity distribution for z=1.5m 4. Numerical simulation for complex thermal environment of building In the summer, big cities seem as a huge heating element, its main source of heat including solar radiation and energy from the consumption of heat release. (Shuzo Murakami,2000) thermal environment analysis more complicated

4 than the wind environment analysis. generally, Solar radiation is the most important factor which needs to consider in thermal analysis. But this kind analysis needs transient analysis, which is a long time job. For dealing with this problem, I use steady-state analysis to solve solar radiation by using UDF. This paragraph will take a real project as an example, to provide you the reference for using steady-state solar radiation analysis in outdoor complex thermal environment. When we use solar radiation analysis, the usually method is using transient analysis. But the analysis time is from one day to one week, maybe much more time is needed for spending. It s annoyed engineers a lot. Now we use a UDF to ask steady-state solar analysis be possible. We deal with a same project with different analysis methods: transient analysis and steady-state analysis. Now we study their differences by contrasting analysis results. As shown in figure 7,which is results contrast for cross-shaped building, they re three groups for different positions contrast. We can find the temperature distribute is very similar between two results and the temperature rise in steady-state is higher than it in transient. The temperature rising positions is the same important in this contrast, as shown in figure8. It s obvious the temperature rising positions are nearly the same in two analysis results. In this contrasting, the temperature in steady-state analysis is about 2-8 degree centigrade higher than it in transient analysis; the temperature rising positions are nearly same in two analysis results; the positions of high and low temperature is also very similar. In conclution, the steady-state solar radiation analysis can be used in real projects and it is structive and meaningful. Figure 7 temperature contrast Figure 8 temperature rising position contrast

5 5. Meshing technology in CFD Meshing is an essential part of the CFD analysis, which is a method to change continuously variable into discrete variables. To discrete control equation in space domain, grid need to be used. (F.J.Wang, 2002) SOFTWARE CRADLE company has been committed to the computational fluid dynamics development in Japan, also was in the international leading ranks for the meshing technology development and application, such as the overlapping grid technology, multiple grid technology, Zooming function, etc. Both of the two practical engineering above use the multiple grid technology. In next, the multiple grid technology will be focused. The meshing method by using general computational fluid dynamics software in architecture analyses often based on uniform grid. The grid is unified handling when it need to be refinement. STREAM grid technology doesn t use uniform grid, and direct to use meshing method based on representative vertex which saves the total number of grid. STREAM senior meshing is named the multiple grid function, can be more effective to keep the calculation accuracy and at the same time to reduce grid quantity. Next is a project based on primary meshing and multiple meshing separately. By the result contrasting, you can find out multiple grid technology has the absolute superiority, with less grid, less computing time and get the same precision. The contrast of different grid division: By using different meshing technique and getting different analysis results, the figure 9 is for primary grid dividing method, the total grid quantity is 3.24 million. Figure 10 is for the use of multiple grid technology, the total grid quantity is 2 million, which is the 2/3 of primary grid quantity. The detail observation area (figure 11) nearly has same grid density, as shown in figure 12. The wind velocity distribution of the two calculation results indicate that two kinds of grid technology keep good consistency in results, as shown in figure 13. Through the contrast analysis we can draw the conclusion that in same accuracy, multiple grid technology uses less grid quantity and less computation time. The calculation time of latter is just a quarter of former, see table 1 below. Figure 9 primary meshing Figure 10 multiple meshing

6 Figure 11 detail observation area Figure 12 grid contrast, left is primary grid, right is multiple grid Figure 13 contrast of velocity contour(z=1.5m) Table 1 parameters contrast Machine specification (1 core);os:windows XP Professional CPU :2.66GHz;Memory: 4.00GB Meshing technology primary meshing multiple meshing Element (million) Convergence cycle Calculation time (min) Memory consumption (GB) Other common evaluation indexes for Architectural CFD analysis

7 To do CFD analysis In architectural design stages, the most important thing is how to evaluate the results. In addition to making the upper limit of wind speed, pressure, temperature and other parameters in some specific area, there still have a common international evaluation indexes in construction industry, such as MRT, PMV, SET, and the age of air. MRT is the abbreviation of Mean Radiant Temperature, which is an average of the radiation temperature used in the analysis of indoor thermal environment evaluation index, is the average temperature refers to the environment all round to human body surface radiation function(toshiaki Ichinose,2005). PMV is the abbreviation of Predicted Mean Vote, that is forecast from the average number of votes(fanger, P.O. 1972). It is an evaluation index for the human body thermal(cold and heat feeling), represents the same hot and cold average feeling of most people in environment. SET(Gagge, A.P. et al. 1986) is the abbreviation of Standard Effective Temperature, which is a temperature in a Standard environment of relative humidity 50%,to be same as hot and cold feeling of human body and the quantity of heat exchange in practical environment equivalent.(standard environment is that: the relative humidity is 50%, wind speed is 0.1 m/s, the human body metabolism is 1.0 met, dressing thermal resistance is 0.6 clo ) Age of air is as one of the important concept to evaluate the ventilation efficiency, shows the time fresh air into the indoor arrived at a point. It can be associated from the concept of age of air "more elder is more dirty because the air is continually pollution by moving." (Shuzo Murakami,2000) The general international evaluation indexes above, you can use them conveniently without programming in scstream. Conclusion: Through wind and thermal environment CFD simulation of several practical engineering in the design phase in this paper, it shows the special CFD analysis tool STREAM in construction industry can help designer check design planning in the design stage and Provide improvement reference data; This paper introduces the steady-state solar radiation analysis is possible to use in practical project.by contrasting, the results of steady-state and transient they have similar results.. This paper also proves the superiority of multiple grid technology, which not only ensure the precision, but also dramatically shorten the calculation time.this also improves the building designers work efficiency and reduces their working intensity. At last, common international construction evaluation indexes is introduced to evaluate the design, provide the tools to the designer for introducing design is up to the standard to the client. REFERENCES Zhu Jia Qi, et al. Shanghai temperature changes and urban heat island,2006. Wang Fu Jun, The computational fluid dynamics analysis-cfd software principle and application, Shuzo Murakami, CFD and Architectural Environmental Design,2000. Toshiaki Ichinose, et al. ATTEMPT TO EVALUATE THERMAL ENVIRONMENT IN THE AREA WITH A LACK OF URBAN SPATIAL INFORMATION

8 DATABASE Fanger, P.O. (1972), Thermal comfort, New York, McGraw-Hill. Gagge, A.P. et al. (1986), A standard predictive index of human response to the thermal environment, ASHRAE Transactions, 92-2B, pp